1. Field of the Invention
The present invention relates to an electrographic apparatus. More specifically, the present invention relates to an electrographic apparatus of reversal developing system wherein discharging or erasing is performed by a discharging light source such as an eraser lamp, and thereafter a surface of a photosensitive member is charged and exposed, and is further developed by toner, and thereafter a sub-charging process such as transfer charging is formed.
2. Description of the Prior Art
One example of an electrographic apparatus of this type is disclosed, for example, in the Japanese Patent Laid-Open No. 104961/1982. In this prior art, movable shield plates are installed at both ends of a transferring corotron so that effective width of discharge of the transferring corotron can be changed in accordance with paper size, and thereby influence of remaining charge of reverse polarity due to the transferring corotron is eliminated.
Also, in another electrographic apparatus disclosed in the Japanese Patent Laid-Open No. 180575/1984, a further charging corotron other than a main charging corotron is installed downstream from the transferring corotron. This charging corotron is for restoring surface potential of the portion of the photosensitive member charged in the reverse polarity by passing through the transferring corotron to the normal potential.
In the first prior art, the effective discharge area of the transferring corotron, that is, the effective transfer area in a direction of width of the surface of the photosensitive drum can be controlled, but control in the direction of advance of the photosensitive drum, that is, in the direction of rotation thereof cannot be performed. This means that even when a plurality of copies are made continuously, intervals are kept without fail between the transfer papers for control, for example, for peeling the transfer papers from the surface of the photosensitive drum. This interval becomes a portion where the transfer paper does not adhere to the surface of the photosensitive drum and the surface of the photosensitive drum passes through the transferring corotron while exposed. Then, the exposed portion is charged by discharge of the transferring corotron, for example, in the negative polarity in the case of reversal development. As a result, in the case where a photosensitive drum which has a discharging characteristic by a photoconductive effect only when the photosensitive drum is charged in a certain polarity is used, if the potential of the surface of the photosensitive drum is intended to be restored near 0 V, the potential cannot be restored to 0 V when it is charged in the negative polarity in such a manner. For this reason, when the photosensitive drum enters charging process intact, the potential cannot be raised to a value at which a good development can be performed. Resultingly, on the portion of the surface of the photosensitive drum left between the transfer papers, a good electrostatic latent image cannot be formed when the same portion becomes an image part in the next copying.
Also, the second prior art has such deficiency that, for example, when portion charged in the negative polarity is intended to be restored to 0 V by a further charger other than a main charger, the potential of the portion charged in the positive polarity from the beginning, that is, the portion passing through the transferring unit with the transfer paper adhering is raised extraordinarily, and the void white phenomenon, so-called pin hole takes place by a breakdown of the photoconductive layer of that portion. Furthermore, since the further charger other than the main charger is provided, a high-voltage power supply and so on are also required accompanying it, resulting in a high cost of the apparatus.